Due to advance of electronic technology, electronic devices, such as computers and mobile communication devices, each using the high frequency band are increasing. For a wiring substrate and a multilayer wiring substrate for high frequency applications to be used for such electronic device, a material having a low specific dielectric constant is generally required. As a resin material having a low specific dielectric constant, there are given, for example, nonpolar polymer resin materials, such as polyethylene, polypropylene, polystyrene, and polytetrafluoroethylene.
However, the above-mentioned resin materials each have a high linear thermal expansion coefficient, which significantly differs from a linear thermal expansion coefficient of a metal wiring material to be formed on the substrate. Accordingly, there is a problem of, for example, peeling or breakage of wiring due to the difference in linear thermal expansion coefficient.
In order to achieve lowering of the linear thermal expansion coefficient of each of the above-mentioned resin materials, there are known techniques each involving utilizing an inorganic substance having a low linear thermal expansion coefficient, such as filling with inorganic powder and glass cloth reinforcement. Meanwhile, the inorganic substance generally has a high specific dielectric constant, and hence there is also a problem in that a specific dielectric constant of a material to be obtained is increased.
In view of the foregoing, there is a proposal of a technology for producing a substrate having a low specific dielectric constant and also having a low linear thermal expansion coefficient through use of hollow inorganic particles, which are particles each having a hollow central portion (Patent Literature 1).
However, when the hollow inorganic particles are blended with a binder, and a paste including the hollow inorganic particles is formed into a certain shape, the hollow inorganic particles are liable to be broken. With this, the water absorption rate of an insulating layer is increased, and hence, when the hollow inorganic particles are used for a substrate material or a wiring board, there is a problem in that dielectric characteristics are reduced. In order to solve such problem, there is a need to expand shells of the hollow inorganic particles. Therefore, there is a limit when an insulating resin composition having a high porosity is to be obtained through use of the hollow inorganic particles.
Meanwhile, there is a proposal that a fluoropolymer dispersion containing a fine silica filler having an average particle diameter of 2 μm or less is formed into a sheet shape through drying and aggregation (Patent Literature 2).
However, there is a need to produce a high-compression molded body owing to the use of the fine silica filler. A high porosity of more than 50% cannot be achieved, and it is difficult to reduce the specific dielectric constant of a material to be obtained.